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<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>


      var str = 'jdfihjfasjdaskdjaosdh'
      var count = 0
      for( var i=0 ; i<str.length; i++){
          if( str[i] === 'j'){
              console.log(i);
              count++
              
          }
      }

       console.log('j出现的次数', count);
       
    </script>
</body>
</html>